如何从WCF客户端应用程序中的UserNamePasswordValidator捕获异常？

I've written simple WCF service using netTcpBinding and security mode="Message" and clientCredentialType="UserName". Everything works fine when I pass valid username and password, session is established the way I wanted. However when the credentials are wrong exception is thrown though I can't catch it in my client application in try catch block. Did anyone have the same problem ? Here is my custom validator...I've written simple WCF service using netTcpBin